This one is driving me up the wall! Hope I can get some suggestions as
to what this problem might be.

Mercruiser 120, 1973. In an attempt to trace an engine knock which is
apparent at revs upto about 2000 then disappears. More a rattle knock
than a heavy knock. Here's what was done to try and eliminate it:

Engine strip down:

Rebored +30, new pistons and gudgeon pins, crank checked by engineering
company (no regrind required) new main bearings and rod bearings
fitted.

Reconditioned cylinder head, new lifters and rods. Skimmed, valve seats
and valves re-cut and ground to perfection, oil seals, valve guides ok.

Carb rebuild kit

New oil pump

All the usual perishables replaced.

All new ignition electrics.

Sometimes the knock (at low revs) sounds so pronounced and mechanical
that it should be something fairly obvious that we can put our hands on
a feel movement. The sound comes roughly from mid height up the engine,
frontish to middle.

There are only the following items left: Fuel Pump and Distributor,
could a rattly knock come from these items? Also, when I take cylinder
3 lead off the sound increases somewhat.

Who's gonna have a crack at diagnosis and rid me of this noise? This
noise does not affect the performance of the engine in anyway and it
starts right on the button. Plenty of power, runs fine, but has this
pain of a rattlling/knock. Drives me up the wall and I'm worried it
might develop into something worse or a bit flys off internally. All
the manifolds are fine and don't leak.

helping suggestions much appreciated.